There is currently a need for powering portable or mobile devices for use in commercial, business, personal, consumer, and other applications. Examples of such devices include cellular telephones, personal digital assistants (PDAs), notebook computers, mobile email devices, music players, radios, compact disk players, video game consoles, digital cameras, electric shavers, electric toothbrushes and even electric cars. Most of these devices include a rechargeable internal battery that needs to be charged by an external power supply or charger, before the device itself can be used. Recently, there has been an interest in providing a universal wireless method for powering or charging one or several mobile devices, batteries, and/or electronics devices. These “wireless power” methods can be generally divided into conductive and inductive methods. The conductive methods use flow of current from a charger and/or power supply into the mobile devices to provide power and receive power through matching contacts on the back of a device and the pad without ‘plugging in’ the device. The inductive methods utilize coils or wires in a charger and/or power supply to create a magnetic field in the vicinity of the surface. A coil or wire in a receiver embedded into or on a device or battery that is in the vicinity of the surface can sense the magnetic field. Power from the charger and/or power supply can be transferred to the receiver without any wired connection through air or other media in between.
Recently, an organization has been started known as the “Wireless Power Consortium” (WPC) which is a business alliance formed to create a universal wireless power charging standard specification for inductive charging. In a wireless power system conforming to the WPC standard specification, the receiver being charged, communicates a “Control Error” signal to a transmitter that is providing charging power to the receiver. The “Control Error” signal provides an indication of a difference between the power being received and the power that the receiver desires. The transmitter generates a proportional integral derivate (PID) response to the error signal and updates the PWM output to provide the desired power to the receiver. Updating of the output has been accomplished in one of two manners traditionally through either frequency modulation or duty cycle modulation. Frequency modulation is applicable in a resonant system in which the power can be controlled by adjusting the frequency of the drive signal. The further that the frequency is from the resonant peak, the lower the delivered power. Duty cycle modulation operates at a fixed frequency (typically near resonance) and controls the output by varying the on-time/off-time ratio.
Both frequency mode control and duty cycle mode control have their own drawbacks and strengths. A drawback for frequency mode control is that the resolution of the frequency output may be limited. The frequency may be set by adjusting the number of system clock cycles in each period. As the operating frequency increases, the resolution of the adjustment decreases. The coarseness of the adjustment may not be accurate enough to meet the system requirements. Secondly, in a consortium defined specification, the supply voltage may be constrained to meet the requirement for the resultant delivered power. This limits the selection of drivers and driver topologies. Other factors besides a specification can also limit operating frequency.
Duty cycle control can have an advantage of higher output resolution compared to frequency mode control. However, with many systems, there is a lower limit at which the duty cycle can be set before the system changes modes of operation or ceases to work completely, such as the transition between continuous and discontinuous modes of operation. To maximize possible output range the duty cycle system is typically run close to resonance, which may result in substantial power for even low duty cycles. Combined with the lower limit to duty cycle, this can easily make the system unable to deliver low power loads. Another issue is that the resonance of the transmitter can shift due to manufacturing tolerances, or environmental changes. If the operating point gets too far from the resonant frequency, high power levels may become unattainable.